Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive Income (Loss), Aggregated by Length of Time Investments (Detail) - USD ($) $ in Thousands	Aug. 31, 2016	May 31, 2016
Investments Debt And Equity Securities [Abstract]
Total investments with unrealized losses, fair value	$ 85,895	$ 89,360
Unrealized losses with a loss position for less than 12 months, fair value	21,014	41,762
Unrealized losses with a loss position for more than 12 months, fair value	64,881	47,598
Total investments with unrealized losses, gross unrealized losses	(7,208)	(10,182)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(1,081)	(4,856)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (6,127)	$ (5,326)